Interferometric optical analyzer and method for measuring the linear response of an optical component

Abstract

An interferometric optical analyzer apparatus comprises a light source, an interferometer and a detection system for determining the linear response, and subsequently any optical parameter, of one or more optical elements using substantially unpolarized light. In one embodiment, the light source supplies substantially unpolarized coherent light over a predetermined range of optical frequencies. The optical element is coupled in one arm of the interferometer and the other arm of the interferometer is used as a reference. The unpolarized light is first passed through the interferometer then through a three-way polarization splitter unit to split the light into at least three light beams according to preselected polarization axes corresponding to three linearly independent states of polarization. The three light beams are coupled to individual detectors and a controller computes Jones matrix elements from the resulting electrical signals. The total polarization independent power may also be determined by an additional detector.

1. An interferometric optical analyzer apparatus for determining the linear response of an optical element (DUT) ( 26 ), comprising:
 light source means ( 10 ) for supplying substantially unpolarized coherent light whose optical frequency is swept continuously and monotonically as a function of time over a predetermined range; 
 interferometer means ( 16 ) having input means ( 18 ) coupled to an output of the light source means ( 10 ), first and second arms ( 20 , 22 , 26 ; 46 ), the first arm ( 22 ) for receiving said optical element ( 26 ) in series therein and the second arm ( 20 ) being a reference path, and output means ( 24 ); and 
 detection means for determining polarization-dependent power (P 1 ,P 2 ,P 3 ) of light at the interferometer output for each of three linearly-independent states of polarization and a fourth power comprising either one of a power (P 4 ) corresponding to a fourth linearly independent state of polarization and a total polarization-independent power (P) of said light at the interferometer output, the optical analyzer apparatus further comprising 
 controller means ( 44 ) for controlling the sweep of the light source means and computing Jones matrix elements (J xx ,J xy ,J yx ,J yy ) from the at least four power measurements (P 1 ,P 2 ,P 3 ,P/P 4 ). 
 
     
     
       2. Apparatus according to  claim 1 , wherein the three unit Stokes vectors representing said at least three different states of polarization form a substantially right-angled trihedron. 
     
     
       3. Apparatus according to  claim 1 , further comprising delay means ( 46 ) in either or both arms of the interferometer. 
     
     
       4. Apparatus according to  claim 3 , wherein the delay means ( 46 ) is adjustable. 
     
     
       5. Apparatus according to  claim 1 , wherein the controller causes the optical frequency of the light source to vary substantially linearly. 
     
     
       6. Apparatus according to  claim 1 , wherein the light source means ( 10 ) comprises a tunable laser ( 12 ) and a depolarizer ( 14 ). 
     
     
       7. Interferometric optical analyzer apparatus for determining the linear response of an optical element (DUT) ( 26 ), comprising:
 light source means ( 10 ′) for supplying unpolarized incoherent light; 
 interferometer means ( 16 ) having input means ( 18 ) coupled to an output of the light source means ( 10 ), first and second arms ( 20 , 22 , 26 ; 46 ), the first arm ( 22 ) for receiving said optical element ( 26 ) in series therein and the second arm ( 20 ) being a reference path, and output means ( 24 ); 
 optical delay scanning means ( 76 ) for analyzing the broadband light according to wavelength; 
 detection means for determining polarization-dependent power (P 1 ,P 2 ,P 3 ), for each of three linearly-independent states of polarization, of light that has passed through the interferometer ( 16 ) and been analyzed by the optical delay scanning means ( 76 ) and determining a fourth power comprising either one of a power (P 4 ) corresponding to a fourth linearly independent state of polarization and a total polarization-independent power (P) of said light at the interferometer output; and 
 controller means ( 44 ) for controlling the scanning of the optical delay scanning means ( 76 ) and computing Jones matrix elements (J xx ,J xy ,J yx ,J yy ) from the at least four power measurements (P 1 ,P 2 ,P 3 ,P/P 4 ). 
 
     
     
       8. Apparatus according to  claim 7 , wherein, in use, the broadband light source means ( 74 ) emits substantially unpolarized light. 
     
     
       9. Apparatus according to  claim 8 , wherein the broadband light source means ( 74 ) comprises an amplified spontaneous emission (ASE) source. 
     
     
       10. Apparatus according to  claim 7 , wherein the broadband source means ( 74 ) emits substantially polarized light and the apparatus further comprises a depolarizer ( 14 ). 
     
     
       11. Apparatus according to  claim 10 , wherein the broadband light source means ( 74 ) comprises one or more LEDs. 
     
     
       12. Apparatus according to  claim 7 , wherein at least one of the broadband light source means ( 74 ) and the optical delay scanning means ( 76 ) exhibits polarization dependency and the light source unit ( 10 ′) further comprises a depolarizer ( 14 ). 
     
     
       13. Apparatus according to  claim 7 , wherein the optical delay scanning means ( 76 ) is disposed before the interferometer ( 16 ). 
     
     
       14. Apparatus according to  claim 7 , wherein the optical delay scanning means ( 76 ) is disposed after the interferometer ( 76 ). 
     
     
       15. Apparatus according to  claim 13 , wherein the optical delay scanning means ( 76 ) comprises a Michelson interferometer. 
     
     
       16. Apparatus according to  claim 1 , wherein the detection means comprises polarization splitting means ( 28 ) for splitting the output signal of the interferometer ( 16 ) into three signals corresponding to said three linearly-independent states of polarization, each of the three signals representing the projection of the input light onto the corresponding analyzer axis, and three photodetectors ( 30 , 32 , 34 ) for detecting the three signals, respectively. 
     
     
       17. Apparatus according to  claim 1 , wherein the detection means comprises polarization selection means ( 66 ) for selecting and outputting, in succession, signals corresponding to said three linearly-independent states of polarization states-of-polarization each of these signals representing the projection of the input light onto the corresponding analyzer axis, and photodetector means ( 70 ) for detecting the outputted signals. 
     
     
       18. Apparatus according to  claim 1 , further comprising means ( 52 C, 54 ) for measuring the input power (P IN ) of the light entering the interferometer. 
     
     
       19. Apparatus according to,  claim 1 , wherein said detection means measures said total polarization-independent power (P) of the light leaving the interferometer, and comprises means ( 24 D, 54 ) for measuring said total polarization-independent power (P). 
     
     
       20. Apparatus according to  claim 1 , further comprising means ( 18 B,  58 ) coupled to the input of the interferometer ( 16 ) for detecting the power (P B ) of backscatter. 
     
     
       21. Apparatus according to  claim 1 , wherein the controller means ( 44 ):
 receives the at least four power measurements (P 1 ,P 2 ,P 3 ,P/P 4 ) and associates each of them with the corresponding one of the four distinct analyzer axes or three distinct analyzer axes and total power, as appropriate; 
 extracts phase and amplitude information from the measurements; and 
 applies a predetermined linear transformation to the phase and amplitude information to obtain Jones matrix elements (J xx ,J xy ,J yx ,J yy ) for the optical element ( 26 ). 
 
     
     
       22. Apparatus according to  claim 21 , wherein said controller means ( 44 ) determines said transformation matrix from the set of analyzer axes before any measurements are made on said optical element. 
     
     
       23. Apparatus according to  claim 7 , wherein the three unit Stokes vectors representing said at least three different states of polarization form a substantially right-angled trihedron. 
     
     
       24. Apparatus according to  claim 7 , further comprising delay means ( 46 ) in either or both arms of the interferometer. 
     
     
       25. Apparatus according to  claim 24 , wherein the delay means ( 46 ) is adjustable. 
     
     
       26. A method for determining the linear response of an optical element (DUT) ( 26 ), comprising the steps of:
 connecting the DUT into a first arm of an interferometer means ( 16 ), a second arm of the interferometer means being a reference path, the first and second arms being connected between input means ( 18 ) and output means ( 24 ) of the interferometer means, an optical length difference between the said first and second arms defining a relative delay between light propagating along the two paths so as to produce interference at the output means; 
 supplying substantially unpolarized coherent light whose optical frequency (υ) is swept continuously and monotonically as a function of time over a predetermined range into the input means of the interferometer means ( 16 ), 
 using detection means comprising analyzing polarizers, detecting three polarization-dependent power values (P 1 ,P 2 ,P 3 ) of three linearly-independent states of polarization, respectively, of light at said output means as a function of optical frequency (υ) and a fourth power value (P/P 4 ) comprising either one of a power value corresponding to a total polarization-independent power value (P) and a fourth linearly-independent state of polarization (P 4 ) of said light at the interferometer output, and 
 computing the Jones matrix elements (J xx ,J xy ,J yx ,J yy ) from the four measured power values. 
 
     
     
       27. A method according to  claim 26 , wherein said four measured signals corresponding to the optical power values (P 1 ,P 2 ,P 3 ,P/P 4 ) provide amplitude and phase information that is minimally sufficient to compute the Jones matrix elements (J xx ,J xy ,J yx ,J yy ) of the said optical element. 
     
     
       28. A method according to  claim 26 , wherein each of said four measured signals has an electrical bandwidth approximately equal to the product of the laser optical-frequency scan speed and the maximal said relative delay. 
     
     
       29. A method according to  claim 26 , wherein the four-element vector |J> representing the elements of the Jones matrix, is determined according to the expression
   | J>=M   −1   |t>   
 
       where M −1  is the inverse of the transformation matrix M representing the known transfer function of the combined interferometer means and detector means, and |t> is another four-element vector whose elements comprise the measured powers P n (υ) expressed in phasor notation. 
     
     
       30. Apparatus according to  claim 1 , wherein the said four measured power values provide amplitude and phase information that is minimally sufficient to compute the Jones matrix elements (J xx ,J xy ,J yx ,J yy ) of the said optical element from the four power measurements (P 1 ,P 2 ,P 3 ,P/P 4 ). 
     
     
       31. Apparatus according to  claim 1 , wherein said detection means has an electronic bandwidth approximately equal to the product of the laser optical-frequency scan speed and the maximal said relative delay. 
     
     
       32. Apparatus according to  claim 1 , wherein said controller means computes the four-element vector |J> representing the elements of the Jones matrix, according to the expression
   | J>=M   −1   |t>   
 
       where M −1  is the inverse of the transformation matrix M representing the known transfer function of the combined interferometer means and detector means, and |t> is another four-element vector whose elements comprise the measured powers P n (υ) expressed in phasor notation. 
     
     
       33. A method for determining the linear response of an optical element (DUT) ( 26 ), comprising the steps of:
 connecting the DUT into a first arm of an interferometer means ( 16 ), a second arm of the interferometer means being a reference path, the first and second arms being connected between input means ( 18 ) and output means ( 24 ) of the interferometer means, an optical length difference between the said first and second arms defining a relative delay between light propagating along the two paths so as to produce interference at the output means; 
 supplying substantially unpolarized incoherent light into the input means of the interferometer means ( 16 ) by way of optical delay scanning means ( 76 ) for analyzing said substantially unpolarized incoherent light according to wavelength, 
 using detection means comprising analyzing polarizers, detecting three polarization-dependent power values (P 1 ,P 2 ,P 3 ) of three linearly-independent states of polarization, respectively, of light at said output means as a function of optical frequency (υ) and a fourth power value (P/P 4 ) comprising either one of a power value corresponding to a total polarization-independent power (P) or a fourth linearly-independent state of polarization (P 4 ) of said light at the interferometer output, and 
 computing the Jones matrix elements (J xx , J xy , J yx , J yy ) from the four measured power values (P 1 ,P 2 ,P 3 ,P/P 4 ).